1. Field of the Invention
In one aspect, this invention relates generally to systems and methods for controlling the behavior or motion of one or more cutting elements to optimize the cutting action of the cutting element(s) against an earthen formation.
2. Description of Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. Conventionally, the drill bit is rotated by rotating the drill string using a rotary table at the surface and/or by using a drilling motor in a bottomhole assembly (BHA). Because wellbore drilling can be exceedingly costly, considerable inventive effort has been directed to improving the overall efficiency of the drilling activity. One conventional measure for evaluating the efficiency of drilling activity is Specific Input Energy (Se), which the drill bit industry defines as the energy required to drill a specific volume of rock in a given time period, i.e. the input energy required to achieve a target ROP.
Generally speaking, drilling efficiency has not changed substantially since industry was capable of estimating or measuring Se. The Se required to drill a volume of rock is strongly influenced by the chip or cutting size generated at the face of the bit. In general Se increases and drilling efficiency declines as cuttings become progressively smaller. This relationship is driven by the amount of energy required to remove a given volume of rock from the parent rock. One can better understand this relationship by thinking of table salt grains vs. kidney beans. For a given volume within a container, more salt grains will be present than beans. It is also evident that more of total volume is contained in fewer beans than salt grains. If one takes a drill cutting the size of the bean and continues to reduce its size until all of its volume is in particles the size of salt grains, it is clear that addition energy has been required. For further illustration, consider a borehole drilled to produce an extremely thin kerf. This could be thought of as a core that is practically the diameter of the final drilled hole. Of course this has practical limits, but does tend to define the largest possible cutting and the minimum amount of energy used to break the core into smaller pieces. In this case drilling efficiency would be maximized from a drill cutting surface to a contained volume standpoint. Said differently, one wants to maximize cutting size and keep the surface area of the cuttings to a minimum; i.e., the cuttings volume to cuttings surface area ratio should be as large as possible.
Herein is the classic method of improving drilling efficiency or reducing Se. The bigger the cutting, the less work done on the undisturbed volume within the cutting. Thus, attempts have been made to increase cutting size to a practical maximum by through design of drill bits and, to some degree, BHA's. Conventional drill bits are provided with a number of cutting elements or cutters on their face. Increased cutting size can be achieved by increasing cutter size, depth of cut, and by increasing bit torque as long as the increased torque produces larger cuttings. There are practical limits to these methods and only limited change to average cutting size has occurred in the past 10 or 15 years.
The present invention address these and other needs relating to the efficiency of drill bit.